1. Field of the Invention
The present invention relates to a thin-film memory element and, also, a method of manufacturing the same.
2. Description of the Related Art
A thin-film memory element comprising a thin-film transistors is disclosed in U.S. patent applications Ser. Nos. 427,041 and 427,252 and EPC Patent Applications Nos. 89/20014.9 and 89/120022.2. Data can be electrically written into, read from, and erased from, this thin-film memory element.
FIG. 1 attached hereto is a sectional view illustrating the thin-film memory element described in the patent application identified above. The element comprises a thin-film transistor of the so-called inverted staggered type. As is shown in FIG. 1, the thin-film transistor is formed on an insulating substrate 11 made of glass or the like. This transistor, which can store data, comprises a gate electrode G formed on the substrate 11, a gate-insulating film 12 formed on the entire surface of the substrate 11, covering the gate electrode G, a semiconductor layer 13 made of i(intrinsic)-type a-Si (amorphous silicon) formed on the gate-insulating film 12 and opposing the gate electrode G, an n.sup.+ -type a-Si layer 14 formed on the semiconductor layer 13, a source electrode S formed on the n.sup.+ -type a-Si layer 14, and a drain electrode D also formed on the n.sup.+ -type a-Si layer 14. The source electrode S and the drain electrode D are connected to two leads (not shown), respectively. The gate-insulating film 12 is made of, for example, SiN. The ratio of silicon atoms to nitrogen atoms, i.e., Si/N, of the film 12 ranges from 0.85 to 1.1. The film 12 can accumulate electrical charge. This is why the thin-film transistor can stored data.
The thin-film memory element shown in FIG. 1 has hysteresis in terms of its V.sub.G -I.sub.D characteristic, where V.sub.G is the gate voltage, and I.sub.D is the drain current flowing in the source-drain path. Due to the hysteresis of the V.sub.G -I.sub.D characteristic, data can be electrically written into, read from, and erased from the thin-film memory element.
FIG. 3 is a graph representing the V.sub.G -I.sub.D characteristic of the thin-film memory element, which has been determined by the use of the measuring circuit illustrated in FIG. 2. As FIG. 3 clearly shows, the V.sub.G -I.sub.D characteristic of the tin-film memory element described above exhibits hysteresis.
The voltage .DELTA.Vth (hereinafter referred to as "threshold voltage") which the thin-film memory element had when a drain current I.sub.D of 1 nA flowed through the thin-film memory element is measured. The threshold voltage .DELTA.Vth-n was -15 V when a voltage of -30 V was applied of the gate electrode G as is illustrated in FIG. 4A. When a voltage V.sub.G of 0 V was applied to the gate G thereafter, a drain current I.sub.D was still flowing via the thin-film memory element Obviously, the thin-film memory element exhibited the characteristic of a depletion-type transistor, which is represented by curve A shown in FIG. 3. On the other hand, the threshold voltage .DELTA.Vth-p was +12 V when a voltage of +30 V was applied of the gate electrode G as is shown in FIG. 4B. In this case, no drain currents I.sub.D flowed in the thin-film memory element unless the gate voltage V.sub.G was higher than 0 V. In other words, the thin-film memory element exhibited the characteristic of an enhancement-type transistor, which is represented by curve B shown in FIG. 3.
Therefore, it suffices to control the voltage applied to the gate electrode G in order to operate the thin-film memory element. More precisely, to erase data from the thin-film memory element, a voltage of -30 V and a voltage of +10 V to the gate electrode G and the drain electrode D, and the source electrode S is grounded respectively, as is illustrated in FIG. 4A, whereby the thin-film memory element exhibits the characteristic of a depletion-type transistor. To write data into the thin-film memory element, a voltage of +30 V and a voltage of +10 V to the gate electrode G and the drain electrode D, respectively, and the source electrode S is connected to the ground, as is illustrated in FIG. 4B, whereby the thin-film memory element exhibits the characteristic of an enhancement-type transistor. In order to read data from the thin-film memory element, a voltage of +10 V is applied to the drain electrode D, the source electrode S is grounded, and a selection voltage of 0 V and non-selection voltage of -20 V are applied to the gate electrode G, as is illustrated in FIG. 4C.
The threshold voltages .DELTA.Vth-n and .DELTA.Vth-p of the conventional thin-film memory element change in the way shown in FIG. 5 as the data is repeatedly read from the memory element. This is because the same voltage is applied to the gate electrode G, not only to write data into the element but also to erase data therefrom. Consequently, it may no longer be possible to read data reliably from the thin-film memory element when data-reading has been repeated a certain number of times.